1. Field of the Invention
The present invention relates to a supercharger control system for an internal combustion engine, and more particularly to a control system for a turbosupercharger.
2. Description of the Prior Art
It has been recognized in turbo-superchargers for automobile engines that a satisfactory supercharging effect cannot be obtained in a low engine speed range because there is very little exhaust gas energy available for the turbine of the supercharger. In view of the problem in Japanese patent application 57-18708 filed on Feb. 10, 1982 and disclosed for public inspection on Aug. 17, 1983 under the disclosure number 58-138222, there is proposed a turbo-supercharger which has a turbine casing formed with a pair of scroll configurations, one of which is provided with a control valve for closing the exhaust gas passage through the one scroll configuration under a low engine speed range. With this arrangement, it is possible to increase the exhaust gas flow speed to the supercharger turbine even under a low engine speed operation, so that a satisfactory supercharging effect can be accomplished. In the system proposed by the Japanese patent application, the control valve may be operated either by an electro-magnetic actuator or by a pneumatic actuator which is operated under the supercharging pressure.
In this type of turbo-supercharger, it is generally recommendable that the control valve for one of the turbine scroll configurations be held in an open position in case of a failure in the control system from the viewpoint of suppressing an excessive rise in the exhaust gas pressure. Thus, in case where a pneumatic actuator is adopted for operating the control valve, the actuator should preferably be of such a type that functions to normally hold the control valve open under an atmospheric condition. When a pressure representing the low engine speed range is applied to the actuator, it should function to close the control valve and further function to open the control valve under a high engine speed range.
It should be noted however that the supercharging pressure as taught by the Japanese patent application is dissatisfactory to accomplish the aforementioned operation of the actuator It may be considered to utilize the exhaust gas pressure for operating the actuator for the control valve, however, the exhaust gas pressure is also dissatisfactory for the purpose. Further, the exhaust gas may give an adverse thermal effect to the actuator.